Patent Document 1 has disclosed a technique that, in a vehicle mounting thereon a belt type continuously variable transmission as a transmission, performs a vertical sliding speed control (or a longitudinal slip speed control) that forcibly returns a belt of the belt type continuously variable transmission to a LOWEST position (a LOWEST transmission ratio position) at a time of vehicle stop. Further, Patent Document 2 has disclosed a vehicle controller that performs a slip-control of a clutch as a frictional engagement element interposed between a rotation driving source as a power source and a driving wheel.
The vehicle mounting thereon the belt type continuously variable transmission as the transmission is generally transmission-controlled (speed-controlled) so that, when starting speed reduction, a transmission ratio having been at a relatively higher speed side is changed to a large transmission ratio of a lower speed side, and the belt is returned to the LOWEST transmission ratio position for a subsequent vehicle start by the time the vehicle stops. However, for instance, when the vehicle speed sharply decreases due to a hard braking, such transmission control (speed control) as described above cannot respond to this sharp decrease of the vehicle speed, then there could be a case where the belt is not returned to the LOWEST transmission ratio position at the time of the vehicle stop. Therefore, as disclosed in Patent Document 1, by applying a high hydraulic pressure to a secondary pulley side which is a component of the belt type continuously variable transmission and forcibly sliding or slipping the belt in a radial direction (in a vertical or longitudinal direction) with respect to an unrotated pulley, the belt is returned to the LOWEST transmission ratio position. This transmission control (speed control) is called the vertical sliding speed control (or the longitudinal slip speed control).
In the technique disclosed in Patent Document 2, when performing an engine-used slip mode (hereinafter called a WSC drive mode) in which the vehicle travels using both driving forces of an engine and a motor while slipping the clutch between the motor and the driving wheel, if a vehicle stop state is judged during the above drive mode, Patent Document 2 carries out a vehicle-stop-time transmission torque capacity correction process that sets a command hydraulic pressure of the clutch so that a transmission torque capacity of the clutch becomes almost zero, i.e. a size of the transmission torque capacity of the clutch gets closer and closer to zero, while performing a learning-control of the command hydraulic pressure of the clutch.
In a case where the belt type continuously variable transmission is employed as a transmission with the technique disclosed in Patent Document 1 being a prerequisite and the vertical sliding speed control disclosed in Patent Document 1 is attempted to be performed, when such vertical sliding speed control as described above is performed in a state in which an input torque of a predetermined value or greater acts on the continuously variable transmission, since the belt is returned to the LOWEST transmission ratio position by forcibly sliding or slipping the belt in the radial direction with respect to the unrotated pulley, slip of the belt and the pulley in a circumferential direction (a rotation direction) also occurs. Durability of the pulley and the belt is therefore decreased. This situation must consequently be avoided.
However, since each component or parts of the continuously variable transmission and the clutch has manufacturing errors and assembling errors, it is difficult to perform the vertical sliding speed control in a state in which a magnitude of the input torque is the predetermined value or smaller. In practice, the vertical sliding speed control tends to be performed in a state in which the magnitude of the input torque is the predetermined value or greater. Because of this, there arises a problem of decreasing the durability of the pulley and the belt as mentioned above, and of lowering response of a subsequent accelerator operation which is caused by the fact that the clutch is disengaged too much.